The present invention relates to a process and apparatus for treating tubular, plastics material net of the type which has been formed by rope-orientating a mesh structure comprising two sets of strands joined together by tenacious intersections, each set extending at an angle to the other set and at an oblique angle to the machine and transverse directions. The invention also relates to net treated in accordance with the process.
One particular application for nets of said type is in the production of tubular packages as used extensively for example in packaging commodities such as agricultural or horticultural produce. In utilizing tubular, plastics material netting in such applications, it has become common practice to form the net tube into discrete packaging "sleeves" each comprising a relatively long length of the tubular net which has been axially bunched. In use, a sleeve is placed over the hollow feed tube of a packaging machine and gradually payed off to form individual packages which receive commodities from the interior of the feed tube. It is clear that the more concentrated the bunching of the tubular net in the packaging sleeve, the greater is the length of netting which can be accommodated on a feed tube of a given length and the longer is the operative packaging time available between sleeve replacements. Accordingly it is desirable in a packaging sleeve for the sleeve material to be concentrated into as short a length as possible.
In one well known kind of plastics material net of said type, the two sets of strands lie in adjacent planes (i.e. bi-planar), the intersections (which may be referred to as junctions) being produced either as confluent, integral (i.e. unbonded) intersections formed before or as the net strands are extruded or may become confluent by adhesive bonding of one extruded strand to another where they cross in contact, the adhesive bond being tenacious due to there being an adequate area of contact between the strands to prevent one strand being torn or stripped from the other under stresses applied to either strand or both for the purpose of stretching and molecularly orientating the plastics material of which the net is composed. in each case, the strands are on casting continuous throughout the net.
It is convenient when extruding such nets to utilize a process and apparatus of the type disclosed in U.K. Patent Specification No. 836,555 in which the net is extruded from a relatively rotating concentric pair of circular dies each having a circular series of spaced extrusions die orifices, the tubular cast product having confluent, integral intersections with the net strands being diversions of the intersections and each set of net strands extending helically of the tubular product so that a diamond-shaped net is produced with each net opening having one diagonal extending in the direction of extrusion (machine direction or MD) and the other diagonal extending transversely thereto (transverse direction or TD).
When producing net of said type in tubular form for use as a packaging net, it is common practice to rope-orientate the net.
In forming packaging sleeves from rope-orientated net of said type, certain difficulties arise. The rope-orientated net is transversely dimensionally unstable, tending to "rope" or "neck" to its minimum diameter. The net must therefore be opened out radially for use on a packaging machine feed tube, and accordingly when forming such nets into packaging sleeves it has been necessary to support the nets in an opened out condition by the use of hollow internal tubular formers made from cardboard, stiff paper or the like. Moreover, such nets do not lend themselves to highly concentrated axial bunching because of the relatively low helix angle of the net strands and the tendency of the net to contract radially.
Knitted net does not have the tendency to "rope" or "neck" and does lend itself to highly concentrated axial bunching because the intersections are not rigid and enable the crossing or intersecting strands to pivot with respect to each other. Thus, although knitted net is inherently more expensive than net of said type, knitted net has found much greater use in the past in producing tubular packages.
U.S. Pat. No. 3,370,116 discloses that tubular, plastic material net of said type can be transversely spread and heat set in this transversely spread condition, which will transversely open out the net. Such transversely opened out and heat set net would be more suitable for forming packaging sleeves than the mere rope-oriented net, but although the net would be more easily bunched axially because of the higher helix angle of the net strands, the net still would not lend itself to highly concentrated axial bunching. It will be noted that although the mandrel shown in the Patent Specification has a converging portion, the net is always under tension from the downstream nip rollers as it passes over the mandrel. Furthermore, it will be noted that although pleats or gussets can be formed, these pleats or gussets run in the machine direction.
German Patent Specification No. 1 173 236 discloses a somewhat similar arrangement, but in this case the rope-orientated net is transversely spread and is then pulled back into a rope of greater bulk which can be used as a yarn; the opened out tube is of relatively small diameter and the net has fine strands, making it normally unsuitable for packaging purposes. Although the mandrel does have a converging portion, the net is always under machine direction tension from the final reel.
German Patent Specification No. 1 303 765 is particularly concerned with a way of forming a tubular, plastics material net. Further, the Specification discloses that the net can be gathered over vertical formers and the drawings show that as the net passes down over the formers, it adopts a transversely corrugated configuration. It is believed that these corrugations will more resemble "bunching" (as defined hereinafter) and that highly concentrated axial bunching cannot occur because of the tendency of the net to spring back into its extended configuration.
The present invention provides a process as set forth in claims 1 and 4, apparatus as set forth in claim 5, and a plastics material net as set forth in claims 14 and 15.
The corrugations occur because the net is not under machine direction tension after being transversely opened out, and the corrugations give the net a bellows-like or concertina-like appearance. Subsequently, highly concentrated axial bunching of the net may be effected, for example for forming the net into a packaging sleeve, and this can be done by hand, at least partly.
By transversely opening out the rope-oriented net and heat-setting in accordance with the invention, the resulting net is found to possess a high degree of transverse dimensional stability so that effectively the tube does not tend to neck or radially contract into rope form as is the case with conventional rope-oriented nets, i.e. the bunched tubular sleeve has no substantial tendency to contract radially inwards into a rope form. Accordingly, the structure lends itself particularly for use in packaging sleeves which, due to the transverse stability of the net, can dispense with the need for an internal support to resist radial contraction. Further, due to the increase in mesh ratio (defined hereinafter) compared with conventional rope-orientated nets, the nets of the invention lend themselves to highly concentrated axial bunching, thus enabling packaging sleeves containing a greater overall length of net per unit length of sleeve to be produced than with previous rope-orientated nets. This effect is enhanced as the mesh ratio is increased, and ideally the mesh ratio should approach a value at which the net strands are substantially transverse. Thus, it is found that for a net produced in accordance with the invention, the structure can be axially bunched to such a degree that the resulting product length is 1/350th of its maximum axially-extended length (defined hereafter and referred to herein as "maximum extended length"). When the net strands approach more closely to a substantially transverse condition, the resulting net structure can by contrast be axially bunched down to a length which is 1/900th of the maximum extended length.
Though the invention could be applied to uni-planar nets, the invention is particularly useful when applied to bi-planar nets.
The following Table illustrates the effect on the reduction of ultimate sleeve length caused by opening out and setting bi-planar nets. Nets 2 to 5 of the Table have been treated in accordance with the invention, and all the nets are from the same rope-orientated starting product, namely net 1.
TABLE ______________________________________ Axial sleeve Length After Axial Bunching/100 Length Meters Maximum Reduction Net No. Extended Length Ratio ______________________________________ 1 38 cm 263:1 2 28.6 cm 350:1 3 21 cm 477:1 4 15.3 cm 656:1 5 11 cm 900:1 ______________________________________
The net of the invention in a highly concentrated axially-bunched form can be particularly suitable for forming packaging sleeves which are made from relatively light gauge rope-oriented nets, for instance nets having a weight of 4.5 to 14 gms/meter in their maximum extended length. In general, it is considered that nets having a weight of below 15 gms/meter in their maximum extended length are of light gauge and having a weight greater than 15 gms/meter are of heavy gauge. Products in such a highly concentrated axially-bunched form when made in accordance with the invention from heavier gauge rope-orientated nets (for instance having a weight around 50 to 60 gms/meter of their maximum extended length) also find application in other uses. For example packages of such nets when slit axially along the tube can be usefully employed as garden net (e.g. anti-bird net) when opened out and such packages can be produced in a more compact shape, due to the highly concentrated axial bunching of the net, than packages of the same overall net content produced from conventional rope-oriented nets.